1. Field of the Invention
The present invention relates to an image sensor used for an image reading portion of a facsimile system, an image scanner, and the like and, more particularly, to an image sensor improved in the driving frequency thereof.
2. Description of the Related Art
An example of the image sensors used for such apparatus as the facsimile systems and image scanners, is a commonly known type which senses reflected light from an original with photodiodes and photoelectrically converts the sensed light to thereby obtain an electric signal corresponding to the image of the original.
In such an image sensor, it sometimes occurs that the output signal therefrom includes so-called offset noise. Since the offset noise causes uneven image density or bad image quality, it is desired that the offset noise is kept as small as possible, Therefore, there have been proposed various techniques to suppress the offset noise. For example in Japanese Laid-open Patent Publication No. 63-28167, there is disclosed an art, in which the noise signal as the cause of the so-called offset noise is picked up separately from the image signal and the picked up noise signal is subtracted from the image signal including the noise and, thereby, an image signal free from the offset noise is obtained.
In the above described image sensor, an amplifier is used for elevating the image signal gone through the subtracting treatment to a desired level. However, since such an amplifier has an optimum operating frequency, one cannot simply increase the image sensor driving frequency according to an increase in the speed of the apparatus using the image sensor and, hence, there have been problems as described below.
A simplified example of the above described conventional amplifier circuit is shown in FIG. 4. With reference to this figure, the problem arising when the driving frequency of the image sensor is increased will be described below.
The circuit shown in FIG. 4 is such that it amplifies an image signal from a photodiode, not shown. The portion in the diagram encircled by a broken line is provided for each bit, i.e., the same is connected to one photodiode so that a signal from the photodiode is input thereto. Since the circuits for each of the bits are the same, the circuit depicted for the first bit will be briefly described below. This circuit has a front-stage amplifier portion 51 formed with an operational amplifier 50 in the center and a back-stage amplifier portion 53 formed with an operational amplifier 52 in the center.
The front-stage amplifier portion 51 constitutes a non-inverting amplifier circuit whose amplification factor is determined by a resistor 54 and a resistor 55. The resistor 55 is provided with a capacitor 56 connected in parallel therewith for cutting off high-frequency noise.
The back-stage amplifier 53 having the operational amplifier 52 constitutes a voltage follower with an amplification factor of 1. Therefore, the total amplification factor of the amplifier circuit for one bit is virtually determined by the amplification characteristic of the front-stage amplifier 51. As is well known, the transient response characteristic of the front-stage amplifier 51 is virtually determined by the time constant as the product of the resistance of the resistor 55 and the capacitance of the capacitor 56. For example, when this circuit is driven by a driving frequency f1 (for example around 1 MHz), if a signal with a rectangular waveform as depicted by the chain line in FIG. 5(a) is applied to the input terminal, a signal waveform as depicted by the solid line in FIG. 5(a) is obtained at the output point A of the front-stage amplifier 51. In such a case, if it is assumed that the output voltage is sampled and input to the back-stage amplifier 53 at the point of time ts when a sufficient time has passed after the inputting of the signal, a voltage difference (.DELTA.V=V1-V2), where V2 is the voltage value at the output point A at the point of time ts and V1 is an ideal voltage value at the same point of time (i.e., the value of the output signal which would be output from the front-stage amplifier 51 when it is assumed to be a so-called ideal amplifier), will become so small as to be practically negligible provided that the driving frequency is within the optimum operating frequency of the front-stage amplifier 51. This is because V2 is the value of the signal when the change in the signal has been sufficiently reduced.
However, when the circuit is driven at a still higher frequency (at a frequency beyond the optimum operating frequency range of the circuit), then, since the transient response characteristic of the circuit itself is the same, it follows that the sampling is performed at a point of time where the output signal has not yet sufficiently risen (refer to FIG. 5(b)). Therefore, the difference .DELTA.Va between the voltage value V2a at the output point A at the sampling point of time ts1 and the ideal voltage value V1, .DELTA.Va=V1-V2a), is no longer negligible.
Therefore, it is considered that the transient response characteristic of the circuit is improved by decreasing the time constant through adjustment of the capacitance of the capacitor 56 in the circuit shown in FIG. 4. Then, the transient response characteristic is improved and the voltage value V2c at the output point A at the point of time ts becomes sufficiently great as shown in FIG. 5(c) and, hence, the sampling error .DELTA.Vc becomes sufficiently small.
However, the decrease in the capacitance of the capacitor 56 decreases the time constant determined by the capacitance of the capacitor 56 and the resistance of the resistor 55, and thereby, as well known, shifting of the cutoff frequency of the front-stage amplifier 51 to the high-frequency side is brought about (refer to FIG. 6). If the cutoff frequency is shifted to the high-frequency side, such results are brought about that the high-frequency noise is increased and the S/N ratio of the image signal is deteriorated. Thus, there has been a problem that the desire for an improved driving frequency of the image sensor and the desire for an improved S/N ratio of the output signal are mutually contradictory.